1. Field of the Invention
The invention relates to a control apparatus and a control method for a vehicle automatic transmission, in which a plurality of gears are selectively achieved, the plurality of gears including a gear that is achieved by engaging a one-way clutch. More specifically, the invention relates to a technology for suppressing a shock when a one-way clutch is brought to a synchronized state.
2. Description of the Related Art
A vehicle, which includes an automatic transmission in which a plurality of gears with different speed ratios are achieved by selectively engaging a plurality of friction engagement devices and a one-way clutch, is available. In the vehicle, power output from a drive power source, such as an engine, is transmitted to drive wheels via the automatic transmission. In the vehicle, when the one-way clutch is brought to the synchronized state (that is, when the one-way clutch, which has been in an idling state, is brought to an engaged state), engine torque is transmitted in a stepwise manner, and thus, the torque is sharply increased. In addition, torque oscillates due to torsional vibration of a drive system. As a result, a shock (hereinafter, referred to “synchronization shock”) may occur.
In order to reduce the synchronization shock caused by bringing the one-way clutch to the synchronized state, for example, a control apparatus for a vehicle automatic transmission described in Japanese Patent Application Publication No. 5-1589 (JP-A-5-1589) executes an engine torque decrease control to decrease an engine output when it is detected that the one-way clutch is about to be brought to the synchronized state at the time of reacceleration. This suppresses a sharp increase in the torque and torque oscillation at a time point at which the one-way clutch is brought to the synchronized state.
In the case where the one-way clutch is in the idling state when the gear is to be achieved by engaging the one-way clutch, a power transmission path in the automatic transmission is not formed, that is, power transmission in the automatic transmission is interrupted, in other words, the automatic transmission is in a so-called neutral state. The automatic transmission may be in the neutral state, for example, when an accelerator pedal is not operated and the vehicle is coasting. Even if a request for acceleration (hereinafter, may be referred to as “an acceleration request”) is made by operating the accelerator pedal when the vehicle is coasting, drive power is not transmitted to drive wheels until the one-way clutch is brought to the synchronized state due to an increase in an engine rotational speed.
Accordingly, it is conceivable to execute a so-called engine torque increase control that increases engine torque to a value larger than engine torque that is normally output according to an accelerator pedal operation amount, in order to decrease a time period from when the accelerator pedal is operated until when the one-way clutch is brought to the synchronized state, to transmit drive power in quick response to the operation of the accelerator pedal (that is, to improve “a drive power response”), in other words, to promote synchronization of the one-way clutch.
FIG. 12 is a time chart showing a conventional control operation performed if an acceleration request is made in the case where the one-way clutch is in the idling state when a gear is to be achieved by engaging the one-way clutch. In FIG. 12, when the accelerator pedal is operated at time point t1, the engine torque increase control is started at time point t2 to promote the synchronization of the one-way clutch before the one-way clutch is brought to the synchronized state. Then, instead of the engine torque increase control, an engine torque decrease control is started to reduce the synchronization shock, at time point t3 immediately before the one-way clutch is brought to the synchronized state. Then, output torque of the automatic transmission starts to be increased at time point t4 at which the one-way clutch is brought to the synchronized state, and the output torque oscillates.